In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these higher device densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. This continuing trend has also led to advanced monitoring and quality control of the semiconductor manufacturing process.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as optical light, x-rays, or an electron beam) through an intervening master template, the mask, forming a particular pattern (e.g., patterned resist). Exposed areas of the coating become either more or less soluble than the unexposed areas (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed with the developer in a developing step. The less soluble areas remain on the silicon wafer forming a patterned coating. The pattern corresponds to the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
Within lithography, patterns are transferred from a mask or reticle onto a photoresist layer, which overlies the film on the wafer through an exposure process. If the mask or reticle consists of defect(s), even submicron in range, such defect(s) may be transferred to a wafer during the exposure. These defect(s) may be generated by the fabrication process utilized to produce the mask or reticle as well as during subsequent handling and processing. Such defect(s) generally fall into two classes: fatal (or killer) defect(s) and nonfatal defect(s).
Critical dimension(s) of the patterned resist, such as line widths, affect the performance of the finished product and are sensitive to processing conditions. Processing conditions that can affect critical dimensions include, for example, conditions relating to resist application, pre-baking, resist exposure, post-baking, and/or resist development. A few degrees variation in the pre-bake temperature, for example, can have a significant affect on critical dimension(s). Many of the conditions that affect critical dimensions can be difficult to control, often resulting in variations from batch to batch.
Controlling these variations within critical dimensions from batch to batch has become crucial in order to achieve higher device densities. Common micro and macro defects within the lithography process fall within the following categories: developer defects (e.g., scumming, developer spots, resist collapse, residue and no develop), contamination (e.g., particles and foreign materials), coating problems (e.g., comets, striations, spin, lifting, splash back and bubbles and no resist coat), focus and exposure defects (e.g., missing fields, focus error, gross misalign, gross blade errors and no exposure), edge-bead removal problems (e.g., missing, wrong width and miscentering), hot spots or focus spots and scratches (e.g., handling errors and tool misadjustment).
For example, contamination can be a serious problem. Particles or foreign materials have the ability to short or cause an open circuit in the formed circuitry upon the micrometer or even sub micrometer silicon wafer. Further, the particle can block processing chemicals from reaching portion(s) of the circuitry on the wafer during processing steps. Some contamination particles can lead to an unwanted electrical bridge from incomplete etching in spaces between lines. In addition, other contamination particle(s) may cause electrical failure due to induced ionization or trapping centers in gate dielectrics.
The categories and examples of defects above are just a few examples of the possible fatal and nonfatal defects. In order to control the possible defects, track systems are used within the industry of lithography. Track systems overcome the limitations of conventional stand-alone systems used in resist application, pre-baking, resist exposure, post-baking, and resist development. Also, track systems allow for easy accessibility of process modules, which reduces maintenance time, consistency of product and increase in productivity.
Techniques, equipment and monitoring systems have concentrated on preventing and decreasing defects within the lithography process. For example, aspects of the resist process which are typically monitored are: the correct mask has been used; resist film qualities are acceptable (e.g., resist is free from contamination, scratches, bubbles, striations, etc.); image quality is adequate (e.g., look for good edge definition, line width uniformity or indications of bridging); critical dimensions are within the specified tolerances; defect types and densities are recorded; and registration is within specified limits. The defect inspection task has progressed into an automated system based on both automatic image processing and electrical signal processing.
Within the lithography process, two automated areas of defect detection have been concentrated upon, electrical signal analysis and image analysis. By using an electrical signal analysis, defects such as opens in circuitry, unwanted electrical bridges and electrical failures can be detected within the silicon wafers. Image analysis can consist of the overlay inspection (OL) and the critical dimension inspection (CD), which are used to determine the quality of the lithography process. The OL inspection measures the registration of consecutive layers of multi-layer semiconductor chips. During the inspection, the wafer is moved to an optical microscope. Under this optical microscope the position of marks or targets of the previous processed layer are measured against the marks of the layer that is currently being added. The CD inspection measures the layer line widths. The layer is moved to a high-resolution CD-SEM (Critical Dimension Scanning Electron Microscopy) where the line width is measured and determined to be within a threshold or pre-determined tolerance.
Even with automatic defect detection systems, defects still arise in which a solution must be found. Solutions to the defects tend to be based on trial and error. A great deal of time and effort must be spent to troubleshoot the cause and to find a fix solution for each defect that may arise. The solution process is manually driven, very objective and hampered by human judgment errors.